The beneficial physical properties of pyrolytic (also called hot-wire or hot-filament) and pyrolytic initiated chemical vapor deposited polymer coatings makes them suitable for a variety of applications.
Current strategies for hydrophobic dielectric polymeric coatings for electrowetting devices are mainly liquid-based protocols that rely on applying a polymer coating solution onto the surface of a substrate with the subsequent removal of the solvent. Coatings applied from the liquid phase, such as fluorinated compounds, silicones, acrylics, urethanes, and potting compounds, are difficult to deposit at the required thickness and can exhibit pinholes and thickness non-uniformity based on wetting effect. Electrowetting devices, however, can handle neither the required weight nor thickness of these coatings nor allow for uneven coverage of high aspect ratio areas of their assemblies.
There are few existing vapor phase approaches to coating electrowetting devices and those that are used possess major drawbacks. The use of parylene (poly(p-xylene)) homopolymeric coatings as the electrically insulating material is one commercialized approach. While the bulk material possesses reasonable dielectric properties, parylene suffers from major issues related to substrate adhesion, especially on substrates composed of multiple materials which are difficult to prime for deposition. As no polymer is truly hermetic, even the best polymer barriers allow some water vapor through. These water molecules compete with molecular segments of polymer encapsulant for device surface sites. Poor adhesion to the substrate permits water vapor to condense at substrate surface sites where parylene polymer segments are poorly adhered. Overtime, these aqueous films subject the device to substantial risk of device failure.
For example, despite parylene's lower water vapor permeability, a 1.5 μm/1 μm bilayer pyrolytic CVD film significantly outperforms parylene coatings during surface insulation resistance testing with interdigitated electrodes used as the test vehicle. This suggests that low water vapor permeability is not the critical parameter in protecting device surface circuitry. Indeed, the critical function of encapsulant coatings is its ability to inhibit aqueous film formation at the surface.
Parylene also possesses significant issues with respect to UV stability. Most parylene compositions display damage due to UV irradiation, including clouding, cracking, and flaking, within 1000 hours of use. Typical design life for many display applications runs in the 25,000-100,000 hour range, meaning that for most of the life of the device, environmental protection may be compromised along with device function (due to decrease light output through a non-transparent barrier layer). Many pyrolytic CVD materials do not possess this issue, such as pyrolytic CVD deposited polytetrafluoroethylene (PTFE) which is transparent in the UV range and consequently entirely unaffected by environmental UV irradiation.
There no currently commercialized vapor deposition approaches for the formation of a material having a lower surface energy than Parylene or variants thereof. Lower surface energy materials are necessary for many electrowetting device applications.
Electrowetting describes the electromechanical reduction of a liquid's contact angle as it sits on an electrically-charged solid surface. As described above, when an electric field is applied across the interface between a solid and a water droplet, the surface tension of the interface is changed, resulting in a change in the droplet's contact angle. In oil ambient (i.e., when the water droplet is surrounded by oil rather than air), the electrowetting effect can provide >100° of reversible contact angle change with fast velocities (>10 cm/s) and low electrical energy (˜100 to 102 mJ/m2 per switch). Electrowetting has become one of the most widely used tools for manipulating tiny amounts of fluids on surfaces. A large number of applications based on electrowetting have now been demonstrated, including lab-on-a-chip devices, optics, and displays.
An important parameter in electrowetting studies is Young's angle (θY), defined as follows:cos θY=(γod−γad)/γao  (1)where; γod is the interfacial tension between the electrowetting liquid (a, typically aqueous) and the oil (o) surrounding the electrowetted liquid; γad is the interfacial tension between (a) and the dielectric layer (d); and γao is the interfacial tension between (a) and (o).
For most electrowetting applications, it is generally desirable to use low voltages (V) to switch from Young's angle to the electrowetted contact angle (θV). Low-voltage operation is particularly important for particular displays, such as e-paper displays, that require very large arrays (thousands or millions) of electrodes. These devices require active-matrix electrode control. Active matrix control makes use of thin film transistors (TFTs) that independently address each of the pixel states. TFTs typically provide reliable operation up to about only 15V. However, achieving reliable electrowetting devices operating at ≦15V has been a considerable challenge.
In an electrowetting system, Young's angle is reduced to the electrowetted contact angle (θV) as predicted by the electrowetting equation,cos θV=(γod−γad)/γao+∈V2/(2dγao)  (2)where: ∈ is the dielectric constant and d is the thickness of the dielectric; γ is used for terms denoting the interfacial tension between the electrowetting liquid, the oil, and the dielectric, as described in equation 1, above; and V is the applied DC or AC RMS voltage.
Once surface tensions are optimized for a high Young's angle (θY), the electrowetting equation predicts that lower voltages may be obtained only by reducing the thickness of the dielectric, or by using a dielectric with a higher dielectric constant. A change in contact angle on the order of 100 degrees is desirable for good electrowetting device function.
The use of high dielectric constant layers, however, has no effect in real-world studies. When such dielectrics are combined with a conventional fluoropolymer top coat, the increased capacitance is substantially masked by the low dielectric constant fluoropolymer. Further, increased capacitance in the dielectric layer results in electric field amplification in the top coat, which can lead to a higher likelihood of charging effects within the topcoat. Some high dielectric constant materials also show excessive leakage currents (due to unwanted electronic carrier injection and transport) when employed in electrowetting systems. It should be noted that most polymeric materials have dielectric constants between 2 and 3, making variation of this parameter even less useful for tuning electrowetting behavior.
Reducing the thickness of the dielectric layer instead (e.g., to the submicron range) has also proven to be a considerable challenge. First, as the dielectric thickness is reduced for a given voltage and target γao value, the magnitude of the applied electric field across the dielectric increases. As such, the probability of charging effects (e.g., ion diffusion into and entrainment within the dielectric) and dielectric breakdown also increases. Furthermore, as the thickness of the dielectric layer decreases, the thickness can eventually match the depth of voids and pin-holes within the dielectric layer. Such defects typically result in immediate dielectric failure when voltage is applied. In this regard, dielectric failure modes for electrowetting are perhaps more complicated than those of purely solid-state systems. That is, under an applied field, the conductive aqueous solution in electrowetting devices is capable of propagating through porous networks within the dielectric. The onset of electrolysis occurs when migrating aqueous solutions reach the electrode, degrading the electrowetting response and progressively destroying the electrowetting surface.
To date, electrowetting products have made use of thick dielectric layers. For example, the first commercial electrowetting products were higher voltage 30-60V, variable-focus electrowetting lenses (Varioptic SA), where thicker (˜3-5 um) parylene dielectrics were utilized. Electrowetting displays also presently use parylene as the dielectric and currently achieve 20V operation, but the operating voltage must decrease further to comply with the needs of conventional TFTs.
Poly (2-chloro-paraxylylene) (trade name Parylene C) is presently the material of choice for optoelectronic electrowetting systems. It is a vapor-deposited conformal dielectric that is commonly employed to provide environmental protection in the microelectronics and medical device industries. Its electrical insulation properties and resistance to water permeation has made parylene one of the most extensively used dielectrics in electrowetting systems (e.g., in Varioptic's liquid lenses). However, because of the onset of leakage current for Parylene C at thicknesses of <1-2 μm, it is not possible to obtain a thin, high-capacitance parylene-C layer for low voltage operation. The failure of parylene to provide adequate insulation at lower thicknesses is attributed to the presence of defects such as voids and pinholes that disappear or are masked when parylene is deposited to greater thicknesses. Further, the long-term hydrolytic stability of parylene C is in question. For example, parylene has been shown to undergo hydrolysis with resultant craze cracking upon exposure to high humidity.
As an alternative to polymeric dielectrics, 100-nanometer-thick inorganic oxide (e.g., Al2O3) films formed using atomic layer deposition (ALD) have shown reliable low voltage (<15V) operation. However, the prohibitive cost of ALD (which is in part due to its very low deposition rates) precludes this option, particularly for larger scale commercial manufacture. In addition, ALD coatings typically require higher deposition temperatures (e.g., 250° C.) that may not be tolerated by some substrate materials. It has been suggested that electric failure could be the foremost issue in long-term reliability for low-voltage electrowetting devices. In addition, these layers are not flexible and therefore do not lend themselves for many embodiments envisioned for electrowetting devices, especially flexible displays and e-paper devices.
A second materials challenge for electrowetting devices lies with the hydrophobic top layer. Hydrophobicity is required to achieve high contact angle modulation. Thus, in typical electrowetting devices, the dielectric layer is overcoated with a thin hydrophobic layer. The hydrophobic topcoat in conventional electrowetting displays is a wet-applied film consisting of a fluoropolymer in a solvent or suspension. Fluoropolymers are known for their exceptional insulating properties. However, wet-applied fluoropolymers have proven to be inadequate for most electrowetting applications. Specifically, wet-applied fluoropolymers do not densify well and can exhibit porous networks that lead to electrical breakdown. This characteristic precludes the dual use of conventional fluoropolymers as the hydrophobic layer and the dielectric layer. Thus, to date, fluoropolymers have typically been used only to impart hydrophobicity. The wet-applied fluoropolymer layer (e.g., Fluoropel) is spin- or dip-coated onto the dielectric layer and subsequently dried and annealed. Dewetting and surface tension effects associated with deposition of these wet-applied coatings can result in poor thickness uniformity and poor conformality to the substrate geometry. This leads to inconsistent contact angle modulation between display pixels. Further, the high temperature annealing step (often at temperatures ≧180° C.) required for many wet-applied fluorocarbon films is incompatible with many substrate materials, especially those proposed for some displays. In particular, the annealing step can deteriorate the dielectric properties of parylene-C (which has a continuous use temperature of only 80° C.). Substrates used in some electrofluidic devices contain fine micro cavities that may be easily plugged by wet-applied coatings.
Finally, for electrowetting applications, the thickness of the top hydrophobic layer must be kept low to (1) minimize the applied voltage needed for contact angle modulation, and (2) to mitigate the potential for charges to be trapped within the top coat. Wet-applied coatings are typically not capable of being applied at less than 30-50 nm thick.
There exists a need for uniformly-deposited hydrophobic layers and thin, non-porous dielectric layers that overcome the limitations described above. Ideally, a single coating technique would be used for deposition of both layers. This would simplify and reduce the cost of device manufacturing. Using vapor deposition for both layers would help ensure coating conformality, uniformity, and low cost. There is also a need for a vapor deposition technique that, unlike ALD coatings, can produce high-integrity coatings at a reasonable rate without damaging the underlying substrate being coated.
Therefore, it is an object of the invention to provide uniformly-deposited hydrophobic layers and thin, non-porous dielectric layers that overcome the limitations of the prior art and methods of making and using thereof.